Drilling at very great sea depths, for example above 2000 meters and notably up to 4000 meters, requires a riser architecture that is different from that of current risers.
What is referred to as a riser is an assembly consisting of a central tube, peripheral lines and possibly other equipments. Such a riser allows fluids to be transferred between the water bottom and an installation that is situated at a higher level, i.e. that can be situated substantially at the water surface or underwater, for example just below the surface.
In fact, these risers are subjected to various modes of vibration, such as lateral, axial or torsional modes. The present invention concerns more particularly the axial modes of vibration and the term "natural period" defines the axial natural period of the riser, or that of an element of the riser.
The invention is particularly well suited for a riser connected by its upper part to a floating installation and whose lower end is free, for example after being disconnected from a blowout preventer or BOP, or from a manifold.
When a riser of great length hangs from a drillship and is free at its lower end, the heave of the ship due for example to the wave motion communicates an excitation thereto in the substantially vertical direction. This excitation can induce high stresses in the riser which can damage it significantly and even lead to its breaking.
This excitation phenomenon, which can be maintained and increased, becomes particularly critical when the natural period of the riser becomes at least equal to the minimum value of a period range for which the floating installation could be excited significantly by the heave.
For example, for a conventional drillship, the period range for which such an excitation has strong repercussions on the riser is above 6 seconds.
The risk of excitation of the ship also exists for the 4 to 6-second range, but it is slighter.
The natural period of a riser notably depends on the following parameters: its linear density m or mass per unit of length, its axial rigidity ES corresponding to the product of the Young's modulus E by the structural section S, and its length L.
The calculation of the natural period of the riser also depends on the geometry and on the dimensioning of the riser, and it is for example described in the article OTC 4317, Offshore Technology Conference, 14th Annual OTC in Houston, Tex., May 3-6 1982.
For a water depth of 4000 meters, the "natural" period of a riser of a conventional type used in the petroleum sphere can reach values of the order of 7 seconds, which are within the period range for which a conventional drillship can be significantly excited by the wave motion.
The excitation phenomenon can increase for example with the number of peripheral lines whose mass contributes to increasing the natural period of the assembly consisting of the riser and the lines.
The prior art thus describes risers comprising notably a central tube and peripheral lines consisting of several elements linked together by slip joints, each one of the elements being immovably fastened to the central tube. The mass of each of the lines thus participates in the mass of the whole riser without participating in its rigidity ES, which leads, in case of great depth, to a value of the natural period of the riser that is great enough for the above-cited problems to be encountered.
Furthermore, the increase in the mass of the riser when the water depth increases leads to the emergence of two phenomena which are of little importance and often disregarded for slight and average water depths but which, in the case of great water depths, become much more important and can condition the dimensioning and the characteristics of the risers. The causes and the effects of these phenomena must be carefully studied.
The increase in the supertensions due to the inertia of the riser during strong storms can lead to a tension decrease and/or to a compression, notably in the upper part of the riser, and induce therein, in correlation with the other motions (surge, sway) and the direct action of the wave motion, crippling bending stresses.
The rise in the natural period of longitudinal or axial vibrations towards values for which the heave amplitude cannot be disregarded can limit considerably, even in relatively calm weather, the operations intended for manoeuvring the riser because of the risks they would present.